Single Sideband (“SSB”) modulators offer an efficient means of upconversion of a signal's frequency. This is because SSB modulators provide a method of upconversion where one of two sidebands are suppressed. This is unlike traditional Double Sideband (“DSB”) upconversion where both sidebands are at similar power levels after the conversion. Compare FIG. 1 to FIG. 2 below:
The apparent difference between the two upconversion schemes is that in the SSB upconversion, the power level of one of the sidebands is lower. The local oscillator (“LO”) bleed through power is also lower in SSB modulation schemes. At the heart of an SSB scheme is an I/Q modulator which takes advantage of the nature of complex signals to suppress the unwanted sideband and LO power during upconversion.
Turning now to FIG. 3, a SSB modulation system 2 typically includes a complex I/Q modulator 4. A complex DAC 6 feeds signals that are nominally 90 degrees out of phase with one another at input of modulator 4. Complex digital to analog converter (“DAC”) 6 receives its signal input from some form of a channel processor 8—in the case shown in the figure, a field programmable gate array (“FPGA”). Channel processor 8 typically performs operations such as placing communication signals in a baseband frequency arrangement, such as, for example, placing multiple communication channels at center frequencies spaced 6 MHz apart.
Even though the unwanted sideband and LO bleed through are reduced using a SSB system, the output, of the I/Q modulator is passed through a high Q (quality factor) filter 10 to ensure specification compliance. At the end of the chain is typically an output stage 12 that places a modulated communication signal, or signals, onto a communication network. It will be appreciated that although FIG. 3 shows stage 12 as an amplifier, the output stage may also include other components, such as, for example, a mixer for further frequency conversion, variable attenuators, or other signal conditioning components. Furthermore, FIG. 3 shows the output of stage 12 coupled to a RF connection port, which may connect the circuitry to the left of it to a network, such as, for example, a cable television network, a cellular telephone network, a satellite communication network, or a over-the-air broadcast network (i.e., an antenna).
Without describing details of the complex math, which one skilled in the art would know, the key to the sideband suppression, and the LO rejection is the balance between the amplitude and phase of the I and Q signals 14, as well as their common mode voltages. One way to optimize these parameters (amplitude, phase and common mode voltage) is to perform a calibration procedure, where corrections are made, and the effects measured with external equipment. The exact gain and phase settings are stored in memory for use in the exact circumstances under which they were determined. This can provide good results, but there will typically be degradation over time due to the effects of aging, temperature, and drift in any associated drive circuitry (DAC gain, LO power, etc.).